Issue No. 2: Chrysomya bezziana
Chrysomya bezziana, the flesh-eating fly, had aroused much public concern recently when cases of human and animals with wounds infested by this insect were reported. This paper aims to provide some introductory remarks on this insect as well as the prevention and control of Chrysomya bezziana infestation.
2. Chrysomya bezziana, the Old World Screw-worm Fly, is an obligate parasite of mammals. The site of infection is usually at superficial wounds.
3. Adult fly feeds on decomposing corpses, decaying matter, excreta and flowers. Adult female only lays eggs on live mammals, depositing approximately 200 eggs at sites of wounding or in body orifices such as the ear and nose. Wounds in the size of a tick bite are sufficient to attract egg-laying. The eggs hatch in 24 hours and the resulting larvae burrow into the host's tissues and feed on the host's dead or living tissue. The larvae are unable to develop in carrion. They leave the wound after 5-7 days and fall to the ground to pupate. Under hot and humid weather, they can complete the life cycle within 15-30 days. Adult flies live on average for 2-3 weeks.
4. This species of fly is widely distributed in the Afrotropical and Oriental regions (including Southern China) extending as far south as Papua New Guinea. It also exists in neighbouring regions of Hong Kong such as Guangdong, Guangxi, Yunnan and Taiwan.
5. In Hong Kong, larvae of Chrysomya bezziana are occasionally found in the wounds of animals including dogs, cattle and pigs. Cases more commonly occurred in the New Territories.
Public Health Importance
6. Chrysomya bezziana is an agent of myiasis: the infestation of live vertebrate animals with larvae, which, at least for a certain period, feed on the host's dead or living tissue. Feeding activity of the larvae may cause serious tissue damage, resulting in loss of condition, injury to the skin/hide, secondary invasion and death.
7. Same as other members of this genus, Chrysomya bezziana adults commonly visit faeces and decaying matters. Because of this habit, they, therefore, are mechanical carriers of pathogens.
8. When fly larvae have infested hosts, larvae can be controlled by removing them from the hosts by forceps. They can also be killed by applying suitable insecticide to the infected areas.
9. Adult fly can be knocked down by the application of insecticides.
10. Improvement in sanitation can help suppress adult flies. Decomposing corpses, decaying matter and excreta are food sources for adult Chrysomya bezziana, and should be disposed of or removed properly. The public should also keep their premises free of accumulation of these matters.
11. When adult control is not possible, fly infestation could be avoided by proofing measures. These include installation of fly screening materials to human and animal dwelling places, dressing of wounds, etc.
Advice to the Public
12. All wounds should be treated immediately. Wound fluids and blood are known to be attractive to Chrysomya bezziana. To avoid myiasis infections, wounds should be cleaned and dressed properly.
13. Animals with suspected myiasis should be brought to the attention of veterinarians.
14. General precautionary measures against filthy flies such as good sanitation and fly proofing should be taken.
Pest Control Advisory Section